130 On the Composition and Properties of Drinking-Water, 
peaty country, and in that case seldom is perfectly clear and 
colourless. 
Besides the visible impurities taken up from the rocky and 
other materials which water meets with in and upon the earth, 
there are others which are held in solution, the presence of which 
cannot be detected by the sense of sight. The brightest, clearest, 
and perfectly colourless spring and river waters are never 
chemically pure ; they all contain in solution a greater or less 
([uantity of saline matter and earthy constituents, which are left 
Ijehind as a fixed residue wlien the water is evaporated to dryness. 
The water which descends as rain, having undergone a species 
of natural distillation, is, if collected in clean vessels and in 
the open country, the purest and softest of all natural waters. 
On evaporation to dryness it scarcely leaves any fixed residue. 
It is contaminated only with exceedingly small quantities of 
carbonic and nitric acid and ammonia, and light floating particles 
of impurities washed by it out of the air during its descent 
as rain. Rain-water collected in towns or smoky localities, such 
as manufacturing or coal-mining districts, contains, in addition 
to the traces of atmospheric impurities just named, soot and 
other mechanical impurities, or constituents dissolved from the 
materials of the roofs of the buildings upon which the rain falls. 
Rain-water collected in towns is always more or less dirty from 
suspended or mechanical impurities, and generally more or less 
yellow-coloured by soluble organic matter. 
In a filtered state rain-water is the softest natural water, and 
most useful for washing purposes or for the feeding of steam 
boilers. It absolutely prevents boiler-incrustations, which cause 
so much inconvenience, when hard waters, largely impregnated 
with lime-salts, have to be used in kitchen boilers and steam 
generators. Rain-water, however, is insipid and wanting in the 
peculiar refreshing taste so much prized in fresh and bright 
spring-water. On keeping, moreover, the organic impurities 
enter into decomposition and impart a disagreeable smell to it, 
which can only be effectually removed by filtration through a 
charcoal filter. ' 
In view of the great advantages of having the command of 
soft water for washing purposes, arrangements should be made 
in every country house for the collection of rain in suitable reser- 
voirs. The rain-water may be gathered in water-tight cemented 
brick-tanks, or it may be stored conveniently in wooden tanks 
or a number of large barrels. But it should not be kept in tanks 
lined with sheet-lead, which would be rapidly corroded ; and 
as this poisonous metal passes into actual solution in the shape 
of oxide of lead, rain-water collected in such tanks should on no 
account be used for drinking purposes. 
